The usual aim in developing a chemical sensor or biosensor is to produce a digital electronic signal, whose magnitude depends on the concentration of a specific chemical or set of chemicals (analyte). The sensor usually includes two main components, a chemical or biological part that reacts or complexes with the analyte in question (ideally specifically) to form new chemical or biological products or changes in energy that can be detected by means of the second component, a transducer. The chemical/biological component can be said to act as a receptor/indicator for the analyte. A variety of transduction methods can be used including electrochemical (such as potentiometric, amperometric, conductimetric, impedimetric), optical, calorimetric and acoustic. After transduction the signal is usually converted to an electronic digital signal.
Since the signal generated by the chemical/biological reaction with the analyte is usually dependent not only on the concentration of the analyte but also on the characteristics of the sensor itself, such sensors usually require calibration before they can be utilised quantitatively. The way in which the signal varies with the analyte concentration determines the shape of the calibration curve (signal versus analyte concentration) and may define the number of calibration points. Typical calibration curves can be straight line, exponential, s-shaped, etc. and the principal of calibration applies to all methodologies of transduction for chemical or biological sensors.
Ideally, the sensor should be calibrated just before its use since some sensor characteristics that can affect the calibration curve vary with time (ageing effect). It is often the case that the time between sensor manufacture and use can be many months, so calibration at the point of manufacture can lead to inaccuracies in the end result.
In the case of a medical sensor, an attendant clinician or nurse may be required to perform the calibration whilst maintaining sterility of the sensor. Additional constraints applied by the clinician/nurse are that the calibration process should be simple to perform, ideally invisible to the person performing the calibration, and be quickly completed (preferably in less than 10 minutes). Calibration of many currently available medical sensors requires the clinician/nurse to carry out a number of specific steps which can lead to errors or inaccuracies in the measurement if the process is not followed correctly.
There is therefore a need for a method of calibrating a sensor which avoids the inaccuracies caused by ageing effects, and which is a simple procedure, capable of being carried out at the point of use by an inexperienced user.